The present invention relates to a control system for alleviating shocks occurring within an automatic transmission.
In an automatic transmission, when a driver places a manual valve from a neutral (N) range to a forward drive (D) range, a hydraulic pressure is supplied to a forward drive friction element, thus permitting establishment of a torque delivery path for forward running of a vehicle with automatic shift, while when he/she places the manual valve from the N range to a reverse drive (R) range, the hydraulic fluid pressure is supplied to reverse drive friction elements, thus permitting establishment of a torque delivery path for reverse running with a single gear ratio. If, upon shifting the manual valve from the N range to the D range or to the R range, the supply of the hydraulic pressure to the corresponding friction element or elements is too quick, the transmission torque of the friction element or elements rises too rapidly to generate substantially great shock. This shock is hereinafter called as a N-D select shock if it occurs upon placing the manual valve from the N range to the D range and as a N-R select shock if it occurs upon placing the manual valve from the N range to the R range. In the case where a friction element is to be engaged with the above mentioned forward drive friction element kept engaged in order to effect a shift depending on running condition of the vehicle, if the supply of hydraulic fluid pressure to the friction element to be engaged is rapid, a great shift shock occurs due mainly to rotational inertia of the engine.
In order to deal with this problem, it has been proposed in U.S. Pat. No. 4,274,308 to use a common accumulator to suppress the above mentioned two kinds of select shock as well as shift shock. This known accumulator is explained along with FIG. 5. It has a stepped piston a biased by a spring b upwards as viewed in FIG. 5. The stepped piston a defines between two different diameter piston ends an intermediate chamber c for receiving a back-up pressure related to the engine load (or engine torque or throttle opening degree). In this known example, a forward drive select pressure, i.e., a line pressure variable in proportion to the engine load and kept supplied to a forward drive friction element during forward drive. It defines a first end chamber d which the smaller diameter piston end is exposed to and a second end chamber e which the larger diameter piston end is exposed to. The first end chamber d receives a reverse drive select pressure, while the second end chamber e receives a second speed ratio pressure.
The operation of the accumulator is hereinafter explained.
When the manual valve is placed at the N range, the stepped piston a assumes the illustrated position under the bias of the spring b since none of the three chambers c, d and e are pressurized. If the manual valve is shifted to the R range, the reverse drive select pressure appears and is supplied to the chamber d in such a manner as to push the piston a against the action of the spring b so that the pressure builds up gradually in accordance with characteristic determined by the spring force of the spring b. This contributes to alleviation of the N-R select shock. If, on the other hand, the manual valve is shifted to the D range, the forward drive pressure appears and is supplied to the chamber c pushing the piston a against the spring b so as to cause a gradual pressure build-up in accordance with the characteristic determined by the spring force of the spring b, thus contributing to alleviation of the N-D select shock. Lastly, if, with the manual valve kept at the D range, the second pressure appears to effect an upshift to the second speed ratio, this pressure is supplied to the chamber e in such a manner as to push back the piston c upwards assisting the action of the spring b, thus alleviating the shift shock occurring during 1-2 upshift.
In order to alleviate the shift shock to a satisfactorily low level with this known accumulator, the setting of the spring b must be such that the piston c begins to move upwards against the forward drive select pressure applied to the chamber c immediately after the supply of second speed ratio pressure to the chamber e has begun. This movement of the piston a must begin even during operating condition with low throttle opening degree setting as will be readily understood from FIG. 6. In FIG. 6, a fully drawn curve .alpha. (alpha) shows the variation in engine specific torque [which is expressed by an equation: (engine output torque)/(the maximum engine output torque)] against the variation in throttle opening degree at 1-2 upshift, while a broken curve .beta. (beta) shows the engine specific torque vs. throttle opening degree at 3-4 upshift.
Besides, the upward movement of the piston a must be carried out against the forward drive select pressure variable in proportion to the throttle opening degree. In order to assure this upward movement of the piston a the spring b which is arranged to assist this movement must have a relatively large spring force.
Since, for the preceding reason, the spring force is large, the N-R select shock or N-D select shock cannot be alleviated until the pressure builds up to a level high enough to push the piston a downwards overcoming the spring force of the spring b. This shick alleviating characteristic is not satisfactory because the select shock is not suppressed to sufficiently low level.
An object of the present invention is to provide a control system wherein an accumulator is operatively disposed therein in order to alleviate not only shift shock, but also select shock to satisfactorily low levels, respectively.